One of the oldest forms of “renewable” energy comes from hydropower. From ancient grain mills to tanneries to hydroelectric generation, humanity has long harnessed the power of flowing water. Unfortunately, increased use of water power has corresponded with increased environmental impacts. Tanneries, for example, were notorious in New England for fouling downstream waters with process by-products. In developing countries, they still do.
With the advent of hydroelectric generation in the late 19th Century, though, the scope of environmental health impacts has expanded dramatically. At present, approximately one-fifth of the world’s electricity comes from hydropower, with China, Canada, Brazil, the United States and Russia leading the generation pack. A typical hydro plant is a system with three parts: an electric plant where the electricity is produced; a dam that can be opened or closed to control water flow; and a reservoir where water can be stored.
Unfortunately, these reservoirs have flooded vast areas — at least 400,000 square kilometres have been lost worldwide. In addition, rivers emerging downstream of a dam may be substantially altered from the character of the river entering an impoundment above a dam. Sedimentation, temperature stratification, and concentration of polluted runoff all affect downstream water quality. From a public health perspective, one of the more alarming effects is the sharp rise in concentrations of toxic chemicals in and below a reservoir following river impoundment. Suspensions and sediments formed from particles settling at the outlet of a reservoir contain higher concentrations of contaminants and have a higher toxicity than suspensions and sediments at the inlet of the reservoir.
Most notably, the reservoir creation process leads to sharp spikes in the methylmercury (MeHg) concentrations of surrounding biota. Methylmercury is an organic molecule produced mainly by bacteria from inorganic mercury naturally present in materials flooded during the course of reservoir creation. It is also the only form of mercury which readily accumulates in fish tissue. Though methylmercury problems in fish are generally confined to the reservoirs themselves and short (<100 km) distances downstream, this contamination can last 20–30 years or more. In addition, fish downstream of dams have higher MeHg concentrations than fish in the reservoir upstream, for a variety of reasons.
Mercury, in either organic or inorganic form, is a potent toxin. MeHg is rapidly absorbed from the gastrointestinal tract and readily enters the adult and fetal brain, where it accumulates and is slowly converted to inorganic Hg.
According to a 2000 study by the National Research Council in support of the EPA & FDA’s mercury rule, exposure to MeHg can result in adverse effects in several organ systems throughout the life span of humans and animals. Some of the documented neurological effects of mercury poisoning include mental retardation, cerebral palsy, deafness, blindness, and dysarthria in individuals who were exposed in utero and sensory and motor impairment in exposed adults. Chronic, low-dose prenatal MeHg exposure from maternal consumption of fish has been associated with poor performance on neurobehavioral tests, particularly on tests of attention, fine-motor function, language, visual-spatial abilities (e.g., drawing), and verbal memory.
The end result of the reservoir-based bioaccumulation process is that downstream populations which regularly eat fish are at a substantially greater risk for mercury poisoning. Consumption of contaminated fish is the major source of human exposure to MeHg in the United States, and mercury contamination is ubiquitous in the American fish population. A recent study of national waterways found mercury contamination in every fish tested; a quarter of the fish studied had mercury levels above safety levels set by the Environmental Protection Agency for people who eat fish on a regular basis.